1. Field of the Invention
The invention relates to a device for detecting inter-cylinder air-fuel ratio imbalance abnormality in a multi-cylinder internal combustion engine, and more particularly to a device for detecting a comparatively large imbalance of air-fuel ratio between the cylinders in a multi-cylinder internal combustion engine and to an abnormality detection method therefore.
2. Description of Related Art
In an internal combustion engine provided with an exhaust gas purification system using a catalyst, it is generally necessary to control a mixing ratio of air and fuel in a mixed gas burned in the internal combustion engine, that is, to control the air-fuel ratio in order to perform highly efficient catalytic purification of hazardous components present in the exhaust gas. In order to perform such control of air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust gas passage of the internal combustion engine and feedback control is implemented such as to match the air-fuel ratio detected by the sensor with a predetermined target air-fuel ratio.
In a multi-cylinder internal combustion engine, the air-fuel ratio control is usually performed by using the same control amount for all of the cylinders. Therefore, the actual air-fuel ratio can vary between the cylinders even when the air-fuel ratio control is implemented. Where the degree of such imbalance is small, it can be absorbed by the air-fuel ratio feedback control and the purification processing of hazardous components contained in the exhaust gas can be performed with the catalyst. As a result, no effect is provided on exhaust gas emission and no particular problem is encountered.
However, where the inter-cylinder imbalance of air-fuel ratio is large, for example, due to a failure of fuel injection system of some cylinders, the exhaust gas emission is degraded, thereby causing problems. It is desirable that such a large imbalance of air-fuel ratio that causes degradation of exhaust gas emission be detected as an abnormality. In particular, in the case of internal combustion engines for automobiles, it is required that the inter-cylinder imbalance abnormality of air-fuel ratio be detected in the onboard state in order to prevent reliably the vehicle from running with degraded exhaust gas emission, and some countries have recently moved to regulate such detection by law.
Since such inter-cylinder imbalance of air-fuel ratio causes torque pulsations, the inter-cylinder imbalance can be detected on the basis of revolution speed fluctuations in the internal combustion engine. In the control device disclosed in Japanese Patent Application Publication No. 2004-52620 (JP-A-2004-52620), the inter-cylinder imbalance is determined from revolution speed fluctuations, and driving comfort is improved by correcting the ignition timing according to the inter-cylinder imbalance.
However, in order to determine the inter-cylinder imbalance of air-fuel ratio with good accuracy on the basis of revolution fluctuations, it is desirable that changes in revolution speed be large during the detection. Meanwhile, since large revolution speed fluctuations adversely affect drivability, a method of advancing the ignition timing separately for each cylinder is used to restrict such fluctuations (ignition timing control conducted for each cylinder separately). However, where the ignition timing control is performed for each cylinder separately, revolution speed fluctuations decrease and the inter-cylinder imbalance of air-fuel ratio is difficult to detect with good accuracy. Thus, it can be said that the detection of inter-cylinder imbalance and ignition timing control conducted for each cylinder separately are in a trade-off relationship with respect to the size of revolution fluctuations, and the problem is that the implementation of ignition timing control conducted for each cylinder separately results in decreased detection accuracy of inter-cylinder imbalance.